Interesting, I just bought an Oppo 105 and I have been trying it with and without my BP6 preamp. So far I would have to say that although the 105 hooked directly to my PP300SST amps sounds great, it tends toward a dry clinical analytical type of sound and just a bit thin on dynamics. When I put the BP6 into the system, it sounds slightly fuller and slightly more musically realistic to me. The difference is subtle though. I am enjoying the sacds and dvd-audio discs I have.
I also have an Apple TV3 hooked to the HDMI input on the 105. This is why I was searching for and asking about a dedicated digital dac/pre.